Madame Forestier is a good friend of Mathilde's who has risen above Mathilde in social status. Despite this discrepancy in their wealth, Madame Forestier does not hesitate to help Mathilde when Mathilde wants jewels to wear to the ball. Madame Forestier shows her generosity by lending Mathilde the jewelry without hesitation. The reader senses, however, that the jewels do in fact mean something to Madame Forestier by her reaction when Mathilde returns the necklace later than planned.
When the two women meet at the end of the story, Mathilde is not the same woman that Madame Forestier once knew. Mathilde has aged and is in ragged clothes, but she decides that she has nothing to lose at this point by telling Madame Forestier the truth about what happened to the necklace. When Madame Forestier reveals that the original necklace was not real, she exhibits a great deal of pity toward Mathilde as she realizes what the couple went through to restore her necklace back to her. This reveals Madame Forestier as compassionate and understanding, which furthers the irony of the story--the reader is left wondering if Madame Forestier would have been mad at all about the original necklace being lost, and wonders if all Mathilde's efforts were for nothing.